In an optical communication system, as the increase of requirements on the system capacity, high-order modulation formats are used to improve the spectrum efficiency. The high-order modulation formats comprise Phase Shift Keying (PSK), Amplitude Shift Keying (ASK), and Quadrature Amplitude Modulation (QAM), etc. Commonly-used QPSK, 16QAM and 8APSK and so on are all examples of high-order modulation formats. In an optical transmitter using a high-order modulation format, an I/Q modulator based on a dual Mach-Zehnder modulator (MZM) is widely used, the structure of which being shown in FIG. 1.
Such an I/Q modulator contains two push-pull MZMs and a phase modulator (Φ). The two MZMs are used to realize modulation of I/Q routes signals, and the phase modulator is used to introduce a fixed 90° phase shift between the I/Q routes. The structure of a single MZM is shown in FIG. 2.
In the MZM shown in FIG. 2, the input optical signals are evenly distributed into two optical waveguides for transmission, and then combined for output. Each of the optical waveguides is applied with electrodes for applying a voltage. The material of the optical waveguides is optoelectronic crystal, and each optical waveguide forms a phase modulator by using an effect of the optoelectronic crystal that its refractive index varies along with an externally applied voltage, with the phase shift generated by the phase modulator to an optical signal being proportional to the applied voltage. A voltage making the phase shift of an optical signal to reach π is referred to as a half wave voltage, denoted by Vπ. The voltages applied on the electrodes of the two optical waveguides of a push-pull MZM are always opposite. The direct current component of an externally applied voltage is referred to as a direct current voltage (or bias voltage), denoted by V, and its alternating current component is referred to as an alternating current voltage, denoted by v. Assuming that the complex amplitude of the light input into a single MZM is Ein=1, the complex amplitude of an output optical signal is:
                              E          out                =                                                            α                2                            ⁢                              ⅇ                                  j                  ⁢                                                                          ⁢                                      π                                          V                      π                                                        ⁢                                      (                                          V                      +                      v                                        )                                                                        +                                          α                2                            ⁢                              ⅇ                                                      -                    j                                    ⁢                                                                          ⁢                                      π                                          V                      π                                                        ⁢                                      (                                          V                      +                      v                                        )                                                                                =                      α            ⁢                                                  ⁢                          cos              ⁡                              (                                                      π                                          V                      π                                                        ⁢                                      (                                          V                      +                      v                                        )                                                  )                                                                        (        1        )            
where, 0<α≦1, α is a positive real number and features loss of each optical waveguide. It is assumed in the above formula that the loss of the two optical waveguides is equal. When direct current bias voltage
      V    =          -                        V          π                2              ,the above formula may be simplified as:
                              E          out                =                  α          ⁢                                          ⁢          sin          ⁢                                          ⁢                      (                                          π                                  V                  π                                            ⁢              v                        )                                              (        2        )            
At this moment, Eout and the alternating voltage v are in a sinusoidal function relation, which is shown in FIG. 3.
  V  =      -                  V        π            2      is the optimal bias point, i.e. light dimming portion of the MZM. Under such a direct current bias voltage, the complex amplitude of the output optical signal varies in phase with the alternating current voltage. FIG. 3 schematically shows waveforms of an alternating current voltage and an output optical signal. Amplitude modulation may be realized by using a single push-pull MZM according to such a characteristic.
As shown in FIG. 1, in an I/Q modulator, in addition that two MZMs are used to respectively realize the modulation of the I route and the Q route, there exists a relative phase shift between the I/Q routes, which may be realized by using a phase modulator, as shown in FIG. 1 in detail. The voltage applied to this phase modulator is a direct current voltage, denoted by VΦ. The complex amplitude of the output optical signal of the I/Q modulator may be expressed as:
                              E          out                =                              α            ⁢                                                  ⁢                          cos              ⁡                              (                                                      π                                          V                      π                                                        ⁢                                      (                                                                  V                        I                                            +                                              v                        i                                                              )                                                  )                                              +                                    ⅇ                              j                ⁢                                                                  ⁢                                  π                                      V                    π                                                  ⁢                                  V                                      ϕ                    ⁢                                                                                                                                      ⁢            α            ⁢                                                  ⁢                          cos              ⁡                              (                                                      π                                          V                      π                                                        ⁢                                      (                                                                  V                        Q                                            +                                              v                        q                                                              )                                                  )                                                                        (        3        )            
For the sake of simplicity, it is assumed that the half wave voltages of each of the phase modulators are equal, denoted by Vπ. However, the conclusion obtained below is not dependent upon such an assumption, that is, such half wave voltages may not be equal, and may be differentiated by different letters in formula (3). VI in formula (3) denotes the direct current bias voltage on the I-route MZM, and vi denotes the alternating current voltage on the I-route MZM. VQ denotes the direct current bias voltage on the Q-route MZM, and vq denotes the alternating current voltage on the Q-route MZM. And VΦ denotes a direct current bias voltage controlling the relative phase shift between the I/Q routes.
In the optimal bias state:
                                          V            I                    =                      -                                          V                π                            2                                      ,                              V            Q                    =                      -                                          V                π                            2                                      ,                              V            ϕ                    =                                    V              π                        2                                              (        4        )            
At this moment, formula (3) may be simplified as:
                              E          out                =                              α            ⁢                                                  ⁢                          sin              ⁡                              (                                                      π                                          V                      π                                                        ⁢                                      v                    i                                                  )                                              +                      j            ⁢                                                  ⁢            α            ⁢                                                  ⁢                          sin              ⁡                              (                                                      π                                          V                      π                                                        ⁢                                      v                    q                                                  )                                                                        (        5        )            
It can be seen from the above formulae that the I/Q modulation may be realized by applying different alternating current voltage on the I-route and Q-route. When the direct current bias voltage on each of the modulators is equal to the value in formula (4), it is the optimal bias state of the I/Q modulator.
In a practical I/Q modulator, the refractive index of an optical wave guide will vary with ambient conditions (such as temperature), which causes the phase shift generated by each optical wave guide to an optical signal to vary with it. Therefore, an I/Q modulator originally located at the optimal bias point is possibly not located at the optimal bias point when the ambient conditions vary, resulting in degradation of the output signals and reduction of system performance. In order that an I/Q modulator is always located at the optimal bias point, the direct current bias voltages VI and VQ on the MZM and the direct current bias voltage VΦ generating the relative phase shift between the I/Q routes (i.e. the direct current bias voltage on the phase modulator) must be adjusted to compensate for the effect brought by the variation of the ambient conditions. To achieve such an object, automatic bias control (ABC) is needed. ABC, as the name suggests, is to automatically adjust bias voltages, so that the whole I/Q modulator always operates at the optimal bias point.
Currently, there exist some ABC methods, which may be classified into three types: 1) methods using pilot signals, such as documents 1 and 2; 2) methods in which no signal is demodulated and control is fed back according to statistical characteristic (such as power) of a signal, such as documents 3 and 4; and 3) methods in which control is fed back according to a demodulated signal, such as document 5.
Document 1: Y. Yin, “Dual-parallel-MZ modulator bias control”, US patent 20070212075.
Document 2: H. Kawakami et al, “Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering”, We.10.P1.47, ECOC 2011.
Document 3: L. Dou et al, “electronic pre-distortion operating at 1 sample/symbol with accurate bias control for CD compensation”, OThT4, OFC 2010.
Document 4: Pak S. Cho et al, “Bias control for optical OFDM transmitter”, IEEE photonics technology letters, vol. 22, no. 14, Jul. 15, 2010.
Document 5: H. G. Choi et al, “modulation-format-free bias control technique for MZ modulator based on differential phasor monitor”, JWA33, OFC 2011.
It should be noted that the above introduction to the background art is only for clear and complete explanation of the technical solution of the present invention, and for the understanding by those skilled in the art. It should not be construed that the above technical solution is known to those skilled in the art as it is described in the background art.